French patent no. 2,963,152 describes a magnetic memory cell as schematically shown in FIGS. 1A, 1B and 1C. FIGS. 1A and 1B below respectively show a sectional view and perspective view of a magnetic memory cell as described in connection with FIGS. 1c-1f, 2a-2b and 3a-3d of French patent no. 2,963,152. FIG. 1C is a simplified top view of this memory cell.
As illustrated by FIGS. 1A and 1B, this memory cell comprises a stack 3 above a conductive track 1. The stack 3 comprises a stack of regions, each of which is formed by a section of a thin layer or a stack of several thin layers. The conductive track 1 is for example formed on a substrate 5 made up of a silicon wafer coated with a layer of silicon oxide and is connected across the terminals A and B. The stacking making up the stack 3 successively comprises, from the track 1, a region 10 made from a nonmagnetic conductive material, a region 11 made from a magnetic material, a region 12 made from a nonmagnetic material, a region 13 made from a magnetic material and an electrode 14. The material of the layer 12 can be conductive; this is preferably an insulating material thin enough to be able to be traversed by tunnel effect electrons. There is a structural difference between the nonmagnetic regions 10 and 12 so as to have an asymmetrical system in a direction orthogonal to the plane of the layers. This difference can in particular result from a difference in material, thickness, or growth mode of these layers.
Lists of materials able to make up the various layers are given in the aforementioned patent application. The magnetic materials of the regions 11 and 13 are formed under conditions such that they have a magnetization oriented orthogonally to the plane of the layers. The magnetic material of the layer 13 is formed under conditions such that it retains an intangible magnetization (trapped layer). The upper electrode layer 14 is connected to a terminal C.
The programming of the memory cell is done by causing a current to flow across the terminals A and B, while a field H oriented horizontally (parallel to the plane of the layers and to the direction of the current across the terminals A and B) is applied. Depending on the relative directions of the current between the terminals A and B and the field vector H, the layer 11 is programmed such that its magnetization is oriented upward or downward.
To read this memory cell, a voltage is applied across the terminal C and one or the other of the terminals A and B. The resulting current across the terminal C and the one of the other of the terminals A and B assumes different values depending on the relative direction of the magnetizations of the layers 11 and 13: high value if the two magnetizations are in the same direction and low value if the two magnetizations have opposite directions.
One characteristic of the memory cell described above is that its programming is done owing to a current flowing across the terminals A and B and a magnetic field applied in the plane of the layers, parallel to the current. No current flows from the terminal A or B toward the terminal C during programming. This has the advantage of completely separating the read and write operations of the memory cell.
Many alternative embodiments are possible. In particular, each layer previously described can be made up of a stack of layers in a manner known in the art to acquire the desired characteristics.
The layer section 10 made from a conductive nonmagnetic material can be omitted, as long as the track 1 is made from a nonmagnetic material suitable for the growth of the magnetic layer 11. The track 1 may then have an overthickness below the stack 3. For the reversal of the magnetization in the layer 11 to be possible, it is also necessary for spin-orbit couplings to be present in the magnetic layer. To that end, it is for example necessary for the layer in contact with this layer 11 (or separated from it by a fine separating layer) to be made up of a material or formed from materials with a high spin-orbit coupling. Another solution is for example for the contact between the magnetic layer 11 and one or the other of the layers 10 and 12 to create this spin-orbit coupling; this may for example be done by hybridization of the magnetic layer 11 with the layer 12 if the latter is made from an insulator (see “Spin-orbit coupling effects by minority interface resonance states in single-crystal magnetic tunnel junctions”, Y. Lu et al. Physical Review B, Vol. 86, p. 184420 (2012)).
It will be noted that the memory cell of FIGS. 1A and 1B can be broken down into two elements: a storage element comprising the track 1 provided with the terminals A and B and the layer sections 10, 11 and 12, and a read element comprising, in the example given above, the layers 13 and 14 and the electrode C. With the same storage element, various read modes could be considered, for example optical reading.
FIG. 1C is a simplified top view of the stack 3. Only the track 1 and the stack 3 are shown, as well as the terminals A and B connected to contacts 15 and 16.
As previously indicated, this memory cell is programmable by applying a current across the terminals A and B simultaneously with the application of a magnetic field having a nonzero component in the direction of the current. Examples of means for generating a magnetic field are given in the aforementioned patent application. The application of an outside field or the production of specific magnetic layers able to create the field H raises practical production problems.
Patent application US 2014/0110004 describes a magnetic memory cell that can be programmed by applying a current without a magnetic field. FIG. 2 is a schematic bottom view of a magnetic memory cell corresponding to FIGS. 15C, 16C and 17C of this patent application. A magnetic field 20 comprises a stack of sections of layers similar to the layers of the magnetic stack 3 described in relation with FIGS. 1A to 1C. The stack 20 has an elongated rectangular shape and comprises, on the side of one of its large sides, a section 22 that, seen from above, extends in a direction orthogonal to the large side. The two opposite ends of the rectangle are connected to terminals A and B by contacts 24A and 24B. The section 22 imparts an asymmetry to the stack 20 that makes it possible to program the memory cell simply by applying a direct current. The direction of flow of the current, from the terminal A toward the terminal B or from the terminal B toward the terminal A, defines the programmed value.
The programming of this memory cell depends on the flow direction of the direct current traversing the device.
It is desirable to have a memory cell whose programming does not depend on the direction of a current flowing between two programming terminals, the current being able to have any polarity or even in particular to be alternating.